It's not AStats that doesn't think you don't own it. It's in a way, steam reporting you don't, by not including the game in the steam API page that is read. (This happens a lot with some of the free to play games, demos and sometimes also with games that are/were on a weekend deal.) It might report you own it again in a few days or hours even. But it also happens with regular games (rarely) and some DLC.

You might ask, then why don't games always stay owned, once owned once. Well that is because of the free weekend deals, where people can play and 'own', according to that same games page, the game for the weekend. And after the weekend the game will be unowned.

Another thing, before steam will report you own a free to play game, it usually only does that after you played/idled the game for at least 15 minutes.

Still the achievements and game will remain linked and the points you have in the game will count for the leaderboard. If you don't want to see the games in your overview anymore, simply go to one of the games you don't own (game page) and press the remove button to remove the game from all your lists.